It is common for aircraft to be required by flight controllers to maintain a racetrack course holding pattern. Regulations well known in the flying art mandate that a pilot enter, fly and exit a racetrack holding pattern in a prescribed manner. These are to insure that mid-air collisions are avoided. FIG. 1 illustrates conventional racetrack flight path holding pattern 10 according to the prior art. Holding pattern 10 is identified by fixed point (FP) 12 having coordinates X, Y, Z where X=latitude or equivalent measure, Y=longitude or equivalent measure and Z=altitude. Thus FP 12 represents a point in space that, according to Federal Aviation Administration (FAA) rules defines a particular location for a holding pattern with compass orientation vector 13 and distance (or time at speed) 21 between turn-points 20, 24 (and 28, 12). Direction 15 is designated as “outbound” and direction 17 is designated as “inbound.” FP 12 is also referred to as the first turn-point (TP1). Distance 21 (or time) is the separation between second turn-point (TP2) 20 and third turn-point (TP3) 24 along outbound leg 22, and equivalent distance 21′ between fourth turn-point (TP4) 28 and fixed point (FP) 12 along inbound leg 30. (TP2) 20 and (TP3) 24 on outbound leg 22 are symmetrical with respect to (TP1) 12 and (TP4) 28 on inbound leg 30. Overall holding pattern length 23 is the sum of distance 21, 21′ plus diameter 19. The holding pattern perimeter is distance 21, 21′ plus the sum of the arc lengths of turns 18 and 26.
Often, holding pattern distances are defined in terms of time at a given speed. Thus, “distance” and “time” are used interchangeably herein to express separations between points in space, it being understood that “time” means time-at-known-speed. It is expected that the aircraft will complete a circuit around the holding pattern in a specified time. Similarly, turn diameter 19 usually results from a “standard rate” turn and thus has a predictable diameter and arc length, but other types of turns can also be used whose diameter and arc length depend on the aircraft characteristics. Such matters are well known in the art. As used herein the words “turn” and “standard rate turn” are intended to include any type of turn.
In the example of FIG. 1, aircraft 14 enters holding pattern 10 on entry path 16 passing through FP 12. This is referred to as a Type II entry because it occurs within angle 27 formed by line 11 and base vector 13 or equivalently, as shown here, between line 11 and inbound leg 30. Angle 27 is 110 degrees and complementary angle 25 is 70 degrees with respect to vector 13. If the entry path lies within angle 25 it is referred to as a Type I entry. Regulation holding pattern entries must lie within angles 25 or 27.
As aircraft 14 passes through FP, (TP1) 12, it rolls onto first turn 18 headed for (TP2). When aircraft 14 completes first turn 18 at second turn-point (TP2) 20, it rolls out onto outbound leg 22 parallel to holding pattern vector 13. Aircraft 14 continues on outbound leg 22 to third point (TP3) 24 where it executes second turn 26 leading to fourth turn-point (TP4) 28 where it rolls back onto inbound leg 30 back toward FP 12, which is also the first turn-point (TP1). The sequence shown by 18, 20, 22, 24, 26, 28, 30, 12 is repeated as long as aircraft 14 remains in holding pattern 10. Historically when the local Air Traffic Control (ATC) released aircraft 14 from holding pattern 10, aircraft 14 proceeded to FP 12 and turned onto exit path 32 originating at FP 12, heading toward waypoint 34 and its next destination, as for example, the local airport landing pattern or another location designated by the ATC.
Nearly all large aircraft have an avionics suite that includes a Flight Management System (FMS) for navigation and other functions and an Autopilot that actually flies the aircraft under the direction of the FMS. Based on navigation and destination information provided by the crew either by manual input or from computer data files, the FMS determines the optimal course and direction to execute various turns and pass through designated waypoints and the Autopilot (AP) issues electronic and/or hydraulic commands to flight surface actuators to steer the plane along the course provided the FMS. The FMS uses a combination of crew inputs, stored information and GPS (or equivalent) aircraft position data to generate steering commands for the AP. Means and methods for doing this are well known in the art. Such systems are commercially available.
Often, holding pattern 10 is flown on autopilot. The aircraft is guided by the Flight Management System (FMS) that keeps track of the aircraft's present position and the holding pattern flight path. Stored in the memory of the FMS is information on the aircraft's flight characteristics, such as for example, its safe turning rate and diameter, rate of climb and descent, and so forth. In general the aircraft flight plan and information on many holding patterns are also stored in the FMS or on computer memory disks that the FMS can access.
Once the FMS is instructed to “Hold” at FP 12 it looks up or calculates each of the successive turn points 12, 20, 24, 28 and provides instructions to the autopilot to execute the required maneuvers to enter holding pattern 10 and keep the aircraft in holding pattern 10 until an “Exit Hold” instruction is received (abbreviated herein as “EH”). When the pilot or navigator toggles or actuates the “Exit Hold” button or command the FMS automatically issues instructions to the autopilot to direct the airplane through FP 12 onto path 32 toward new waypoint 34 and its next destination. Means and methods for determining aircraft location and performing such calculations and maneuvers are well known in the art. Alternatively, if the AP is not being used and the pilot is actively flying the aircraft, the FMS can provides the pilot with information on when a given turning point is reached and the required course change, that is, the FMS can lead the pilot around the holding pattern and informs him or her when and how to exit.
Historically the FAA required that aircraft 14 pass through FP 12 on entering and exiting holding pattern 10 and that it enter within angels 25, 27 and exit along path 32. This has the disadvantage that if the aircraft, for example, is on turn-one 18 when the depart or exit holding pattern message is received from the ATC and the “Exit Hold” (EH) command given to the FMS, the aircraft must still fly the entire sequence 18, 20, 22, 24, 26, 28, 20 back to FP 12 before it can exit holding pattern 10. Further, even if the aircraft is on inbound leg 30, being constrained to exit along path 32 through FP 12 can require additional distance and time. Additionally the limitation that entry into holding pattern 32 must be through FP 12 can also require additional time and flight distance. These limitations all waste time and fuel. The FAA has now relaxed these requirements for certain types of holding patterns designated, for example, as Types HA or HM.
Accordingly, an improved means and method is needed for the aircraft FMS to determine an exit pattern using the least time and distance depending upon where the aircraft is in the holding pattern when the “Exit Hold” (EH) command is given and consistent with flight safety. In addition, different entrance paths and circumstances need to be accommodated. Further, it is desirable that the aircraft's performance characteristics be taken into account. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.